SearcharxivSearch

arXiv subjects

Giovanna Fragneto

Publications and source records attributed to Giovanna Fragneto.

8 recordsLinked to original sources

Protein Dynamics Beyond Structure Prediction

The ability to predict protein three-dimensional structures from amino acid sequences is a landmark achievement in molecular biology, where recent deep learning approaches such as AlphaFold are the culmination of decades of work. Yet, the quantitative understanding of how protein sequences give rise to dynamic conformational changes and higher-order assemblies remains unsolved. Folding and conformational states are dynamic, stochastic processes, shaped by sequence, energy, co-translational constraints, chaperone machineries, and the physicochemical conditions of the cellular environment. Recent advances now position the field to move beyond static structural endpoints toward a mechanistic understanding of folding dynamics in living systems. Single-molecule techniques enable time-resolved observation of folding trajectories and intermediate states hitherto hidden by traditional structural biology approaches, while computational innovations and data-driven approaches offer new ways to integrate heterogeneous data across scales. In this Roadmap, we review the current conceptual landscape of protein folding, examine the experimental and theoretical gaps that remain, and discuss emerging strategies that integrate high-resolution measurements with multiscale modeling. We outline a roadmap toward a quantitative and predictive science of protein folding dynamics, conformational kinetics, and macromolecular self-assembly. Realizing this vision would transform our understanding of the dynamics of molecular self-organization, from the folding of individual polypeptides to the emergence of dynamic macromolecular complexes. This will enable rational control of folding and misfolding in health and disease, extend protein engineering principles beyond static structural design, and establish a mechanistic foundation for predictive and personalized interventions in proteostasis-related disorders.

q-bio.BM

The role of confined water in the emergence of electrostatic strong coupling as revealed by nanoseparated charged lipid layers

This study investigates the interplay between Strong Coupling (SC) attraction and hydration repulsion in nanoconfined water between like-charged phospholipid layers. It challenges the assumption that SC attraction requires multivalent counterions by showing that hydration water can enhance electrostatic interactions. We combine reflectivities with numerical simulations to analyze supported phospholipid layers under different relative humidity and surface charge densities. X-ray fluorescence demonstrates that we can control the valence of the associated counterions. Experimental measurement of the water thickness, combined with precise determination of charged surface positions by numerical simulations, enable us to compare our experiments with a theoretical model. It shows that charge-screening by hydration water induces SC attraction, even at moderate surface charge densities with monovalent counterions. Furthermore, hydration repulsion is stronger for DPPS compared to DPPC. These findings offer insights into the forces that control interactions between phospholipid layers and have important implications for biological and colloidal systems.

cond-mat.soft

Attractive Interaction between Fully Charged Lipid Bilayers in a Strongly-Confined Geometry

We investigate the interaction between highly charged lipid bilayers in the presence of monovalent counterions. Neutron and X-ray reflectivity experiments show that the water layer between like-charged bilayers is thinner than for zwitterionic lipids, demonstrating the existence of counterintuitive electrostatic attractive interaction between bilayers. Such attraction can be explained by taking into account the correlations between counterions within the Strong Coupling limit, which falls beyond the classical Poisson-Boltzmann theory of electrostatics. Our results show the limit of the Strong Coupling continuous theory in a highly confined geometry and are in agreement with a decrease in the water dielectric constant due to a surface charge-induced orientation of water molecules.

cond-mat.soft

Reduction in Tension and Stiffening of Lipid Membranes in an Electric Field Revealed by X-ray Scattering

The effect of AC electric fields on the elasticity of supported lipid bilayers has been investigated at the microscopic level using grazing incidence synchrotron x-ray scattering. A strong decrease in the membrane tension up to 1mN/m and a dramatic increase of its effective rigidity up to 300kBT are observed for local electric potentials seen by the membrane < 1V. The experimental results were analyzed using detailed electrokinetic modeling and non-linear Poisson-Boltzmann theory. Based on a modeling of the electromagnetic stress which provides an accurate description of bilayer separation vs pressure curves, we show that the decrease in tension results from the amplification of charge fluctuations on the membrane surface whereas the increase in bending rigidity results from direct interaction between charges in the electric double layer. These effects eventually lead to a destabilization of the bilayer and vesicle formation. Similar effects are expected at the tens of nanometer lengthscale in cell membranes with lower tension, and could explain a number of electrically driven processes.

cond-mat.soft

The component groups structure of DPPC bilayers obtained by specular neutron reflectometry

Specular neutron reflectometry (SNR) was measured on a system of a floating bilayer consisting of 1,2-dipalmitoyl-d62-\textit{sn}-glycero-3-phosphocholine (d62-diC16:0PC) deposited over a 1,2-dibehenoyl-\textit{sn}-glycero-3-phosphocholine (diC22:0PC) bilayer at 25 and 55 °C. The internal structure of lipid bilayers was described by a one dimensional scattering length density profile (SLDP) model, originally developed for the evaluation of small angle scattering data. The corresponding model reflectivity curves successfully describe the experimental reflectivity curves of a supported bilayer in the gel phase and a system of a floating bilayer in the liquid crystalline phase. The reflectivity data from the supported bilayer were evaluated individually and served further as an input by the data treatment of floating bilayer reflectivity curves. The results yield internal structure of a deposited and floating bilayer on the level of component groups of lipid molecules. The obtained structure of the floating d62-diC16:0PC bilayer displays high resemblance to the bilayer structure in the form of unilamellar vesicles, however, simultaneously it shows rate of fluctuations in comparison to unilamellar vesicle bilayers.

cond-mat.soft

Controlling interactions in supported bilayers from weak electrostatic repulsion to high osmotic pressure

Understanding interactions between membranes requires measurements on well-controlled systems close to natural conditions, in which fluctuations play an important role. We have determined, by grazing incidence X-ray scattering, the interaction potential between two lipid bilayers, one adsorbed on a solid surface and the other floating close by. We find that interactions in this highly hydrated model system are two orders of magnitude softer than in previously reported work on multilayer stacks. This is attributed to the weak electrostatic repulsion due to the small fraction of ionized lipids in supported bilayers with a lower number of defects. Our data are consistent with the Poisson-Boltzmann theory, in the regime where repulsion is dominated by the entropy of counter ions. We also have unique access to very weak entropic repulsion potentials, which allowed us to discriminate between the various models proposed in the literature. We further demonstrate that the interaction potential between supported bilayers can be tuned at will by applying osmotic pressure, providing a way to manipulate these model membranes, thus considerably enlarging the range of biological or physical problems that can be addressed.

cond-mat.soft

Effect of an electric field on a floating lipid bilayer: a neutron reflectivity study

We present here a neutron reflectivity study of the influence of an alternative electric field on a supported phospholipid double bilayer. We report for the first time a reproducible increase of the fluctuation amplitude leading to the complete unbinding of the floating bilayer. Results are in good agreement with a semi-quantitative interpretation in terms of negative electrostatic surface tension.

cond-mat.soft

Swelling of phospholipid floating bilayers: the effect of chain length

The equilibrium distance between two lipid bilayers stable in bulk water and in proximity of a substrate was investigated. Samples consisted of a homogeneous lipid bilayer, floating near an identical bilayer deposited on the hydrophilic surface of a silicon single crystal. Lipids were saturated di-acyl phosphocholines, with the number of carbon atoms per chain, n, varying from 16 to 20. The average and r.m.s. positions of the floating bilayer were determined by means of neutron specular reflectivity. Samples were prepared at room temperature (i.e. with the lipids in the gel phase) and measurements performed at various temperatures so that the whole region of transition from gel to fluid phase was explored. Data have been interpreted in terms of competition between the interbilayer potential and membrane fluctuations and used to estimate the bending rigidity of the bilayer.

cond-mat.soft